<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 
            class 可以重复
            id   具有唯一性(只有一个id名，一个页面id不能重复)
        */

        #a {
            color: blue;
        }
        .b {
            color: red;
        }
    </style>
</head>

<body>

    <div id="a" class="b">hi</div>
    <div id="a">hi</div>

</body>

</html>